The present invention relates to improved solutions for etching titanium-tungsten mixtures, and nitrogen-stuffed versions and sandwich layers of same. More particularly, the present invention relates to improved etchants which include at least one oxidizing agent and one fluoride salt.
Titanium-tungsten (TiW) is a well-known adhesion and diffusion barrier. It is commonly used as a barrier metal to prevent cross-diffusion of aluminum with either silicon or gold. These materials find particular utility in integrated circuit fabrication. Nitrogen stuffed versions of titanium-tungsten mixtures (TiW(N)) are even better diffusion barriers and are oftentimes produced by reactively sputtering or depositing titanium and tungsten under a partial pressure of nitrogen. A third titanium-tungsten barrier layer consists of a combination or sandwich of layers, typically TiW-TiW(N)-TiW.
Articles which describe the use of TiW include "Studies of the Ti-W/Au Metallization on Aluminum," Thin Solid Films, Vol. 53, 1978, pp. 195-200. The use of TiW in chip interconnect metallization is described in "Reliability of High Temperature I2L Integrated Circuits," IEEE/IRPS Proc. 1984, Intl. Rel. Phys. Symp., pp. 30-36. The use of TiW for TAB (tape-automated-bonding) wafer bumping is described in "Metallurgy of TiW/Au/Cu System for TAB Assembly," J. Vac. Sci. Technol., May/June 1985, pp. 772-76. In addition, U.S. Pat. Nos. 4,927,505 and 4,880,708 describe the use of TiW and TiW(N) as an adhesion and barrier metallization for TAB wafer bumping. U.S. Pat. No. 4,486,946 describes the use of TiW as a barrier in silicon semiconductor processing of NPN devices. These references are incorporated herein by reference.
Equally well-known, however, is that titanium-tungsten is very difficult to etch due to the different chemical properties of the two metals. Especially difficult to etch is the combination of layers (TiW-TiW(N)-TiW). This is because of the different etch rates of the layers. This difficulty is detrimental in many applications since it may lead to undercutting of patterns, i.e., excessive removal of material in the horizontal or lateral direction which reduces the size of the patterns.
One method of etching involves the use of a dry etch using flourine-based gases. U.S. Pat. No. 4,782,032 describes a process for making field-effect transistors using TiW(N) as the barrier metal. That reference describes the use of a flourine-based plasma in patterning the film. U.S. Pat. No. 4,849,376 describes the use of a dry etch process which uses flourine-based gas as an etchant for TiW in the fabrication of GaAs field-effect transistors.
Dry etchants find particular utility when precise etching is required. Dry etching, however, is expensive due to the high capital cost of reaction ion etch (RIE) systems and are limited in application because they require a hard mask of nickel, aluminum or gold for RIE patterning. Further, for TiW(N), dry etching is difficult to do, especially if selectivity is desired over silicon, silicon oxide, or silicon nitride.
Another etching method involves wet chemical etching. Numerous wet etchants, many of which are commercially available, exist for etching titanium and tungsten individually. In contrast, however, to date, only two wet etchants have been identified that remove mixtures of titanium and tungsten. The most commonly used etchant for TiW is hydrogen peroxide, H.sub.2 O.sub.2. U.S. Pat. Nos. 4,814,293 and 4,787,958 disclose hydrogen peroxide etching solutions for TiW. Similar teachings are found in U.S. Pat. Nos. 4,740,485; 4,491,860 and 4,711,701. These etchants, however, remove TiW(N) poorly and slowly. In addition, the shortcomings of the etchants are particularly evident when TiW-TiW(N)-TiW sandwiches are used since the differential etch rates among the layers cause severe undercutting of masked patterns. Also, these H.sub.2 O.sub.2 etchants generally have short shelf-lifes and use-lifes since they are known to decompose readily.
The present inventor previously discovered that additions of ammonium hydroxide, NH.sub.4 OH, to hydrogen peroxide accelerates the etching of TiW(N) at a higher rate of increase compared to TiW, and precise mixtures of H.sub.2 O.sub.2 and NH.sub.4 OH can be used to match the etch rate of both TiW and TiW(N). This approach, however, requires precise control of the nitrogen contents in TiW(N) and the concentrations of H.sub.2 O.sub.2 and NH.sub.4 OH, since small variations in either have significant, potentially negative influences on the etching ability of the solutions.
Another chemical system that removes TiW and TiW(N) is a solution of nitric acid and hydrofluoric acid, HNO.sub.3 -HF. This system can etch both TiW and TiW(N) quickly and cleanly; however, its use in integrated circuit manufacturing is undesirable since it attacks silicon, silicon oxide, silicon nitride and aluminum.
The present inventor also previously discovered that the addition of isoctylpolyethoxyethanols, such as nonoxynol-9 and -10, mixed in 1 part HF, 10 part HNO.sub.3 and 25 part water reduces the attack of these acids on silicon and its compounds; however, its attack of aluminum is not deterred. Thus, the HNO.sub.3 -HF system has little use as a nondestructive etchant for TiW or TiW(N).
Accordingly, there exists the need for an improved etchant solution for TiW, TiW(N), and combination layers thereof.